O2 transport to heart and skeletal muscle is studied using myoglobin O2 saturation to determine intracellular pO2. Measurements are made on individual muscle cells, at rest and under various conditions of stress, by means of a microspectrophotomter. Measurments of pO2 are correlated with direct measurements of intercapillary distance. Capillary density intercapillary distance capillary diameter, and the functional capillary reserve are compared in normal and hypertrophied rat hearts under various conditions of O2 supply. The contribution of intrinsic neurones and of vasodilator metabolites to post-contraction and post-occlusion vasodilation is studied.